Tungsten (W) is a common metal used for conductors on a silicon wafer. A tungsten layer is typically formed using a CVD process where WF6+SiH4 (silane) gases react to leave a layer of tungsten on a surface. Tungsten is not directly deposited on silicon since the fluorine in the WF6 reacts with the silicon to degrade the adhesion of the tungsten layer and etches the silicon surface. Additionally, the W atoms will migrate into the silicon.
It is known to form a thin oxide layer on the silicon surface and then form the tungsten layer over the oxide. This results in a thick tungsten layer, a thin WOx interface layer, and a thin oxide layer beneath the surface of the wafer. The WOx and oxide layers are highly resistive, resulting in poor electrical contact between the tungsten layer and the silicon.
Accordingly, a non-tungsten barrier layer (e.g., Ti) is typically formed between the tungsten layer and the silicon, as shown in FIG. 1.
FIG. 1 is a cross-sectional view of one application of using tungsten to electrically contact silicon. An oxide layer 10 formed over a silicon substrate 12 is etched to expose a portion of the silicon. The exposed portion may be a doped region of an electronic device, such as a transistor or diode, or any other silicon region. A multi-layered barrier layer 14 of Ti/TiN is first formed over the silicon to improve adhesion of the overlying tungsten layer, protect the silicon from fluorine during the subsequent WF6 exposure, and block W atoms from migrating into the silicon.
A conventional CVD tungsten deposition process is then performed to form a relatively thick layer of tungsten 16. The tungsten layer may be patterned and etched for defining a conductive plug for being contacted by a subsequent aluminum layer or for otherwise forming a conductor to other components.
Forming the Ti/TiN barrier requires steps that significantly add to the cost of processing the wafer.